1. Field of the Invention
The present invention relates to an estimation apparatus for estimating the state of a vehicle located in a frontward field, and in particular, the estimation apparatus which involves, for the estimation, the transmission of radar waves toward a field and the reception of radar waves reflected from objects, such as vehicles, located in the field.
2. Description of the Related Art
Recently, various measures for driving safely have been provided. One such measure is to estimate the travel states of vehicles which are located ahead. Japanese Patent Laid-open Publication No. 2002-99986 discloses an apparatus which is able to provide such measures. This publication provides a radar apparatus that uses radar waves to estimate the travel state of a vehicle located apart from another vehicle.
In the configuration of this publication, the radar apparatus is arranged on a straight road and configured to periodically emit millimetric waves toward a given directional range. This radar apparatus receives radar waves reflected from a vehicle and estimates the travel state of the vehicle based on information derived from the received radar waves. Specifically, it is assumed that the vehicle is in a linear travel motion, so that the Kalman filter designed based on a linear motion model is used to calculate the speed and acceleration of travel of the vehicle. These calculated results are processed as information predicting the motion state of the vehicle. Using the radar apparatus, it is possible to detect the current position of the vehicle which travels on the road and, if a vehicle on the road is in a travel motion, it is also possible to predict the vehicle's accident states in the future. Accordingly, in this radar apparatus, both observed values and predicted values in the future raise the accuracy of determining whether or not the vehicle is stopped on the road and the accuracy of calculating the vehicle speed, whereby it is helpful for improving prediction of vehicle-to-vehicle collisions and urging drivers or vehicle systems to prevent such accidents.
However, the above-described estimation apparatus is confronted with two difficulties. One difficulty is that it is difficult to accurately detect information indicative of the size and direction of a vehicle located in the frontward field (hereinafter, simply referred to as “located ahead”). In this case, “accurately” means a degree of accuracy necessary to reliably prevent collisions between vehicles. Since the shape of a vehicle is unknown, it is very difficult to reliably predict the collisions. The other difficulty is that it is impossible to predict the motion of a vehicle when the vehicle turns. That is, in the case of the foregoing conventional foregoing apparatus, the future state of a vehicle is predicted under the assumed condition that the vehicle travels straight. Thus, the linear motion model is used for the prediction on the Kalman filter. This means that it is difficult or almost impossible to grasp the vehicle's turning motion, thus lowering reliability of the prediction.